The present invention relates to a novel polymer, polyvinyl methylphosphinic acid, to a process for its preparation and to its use in the production of offset printing plates.
Organic polymers which are substituted by phosphorus-containing groups, and processes for their preparation are already known. Thus, the preparation of liquid to solid polymers from vinylphosphonic acid dichloride is described in German Pat. No. 1,032,537 (=British Pat. No. 865,046). German Pat. No. 1,106,963 concerns the polymerization of vinylphosphonic acid to give polyvinylphosphonic acid which is obtained as a very viscous to hard, clear and nearly colorless product. It is readily soluble in water, in alcohols and in polyhydric alcohols, such as glycols, glycol ethers, polyglycols or glycerol, and in other solvents, giving more or less viscous solutions. Polyvinylphosphonic acid also possesses marked film-forming properties.
Polymers of this type formed from vinylphosphonic acid have considerable importance as intermediate products for plastics, and also in the preparation of flameproofing agents, textile auxiliaries and wetting agents and emulsifiers. They are also used in the production of coatings on metal surfaces for the purpose of protecting the latter from corrosion (cf. German Pat. No. 1,187,100). Furthermore, polyvinylphosphonic acid has acquired considerable importance in the production of planographic printing plates (offset printing plates).
Plates of this type comprise a layer support on which a radiation-sensitive reproduction layer has been applied, with the aid of which an image of an original is produced by photomechanical means. After the printing form has thus been prepared, the layer support carries the ink-receptive image areas, and, at the same time, on the image-free areas, forms the water-receptive background to the image (non-image areas).
What is required, therefore, from a layer support which is to be suitable for light-sensitive material for the production of a printing plate, is, on the one hand, that the image areas, which have been developed from the copying layer of the material and are to be printed, should adhere very firmly to the latter and, on the other hand, that the layer support constitutes a hydrophilic image background and loses nothing of its effectiveness for repelling oleophilic printing inks under the manifold requirements of the printing process. Therefore, the layer support must also have a surface structure which is always porous to a certain extent, so that its surface can retain sufficient water in order to have a sufficiently repellent action against the printing ink used in the printing process.
The base material used for layer supports of this type can be aluminum, steel, copper, brass or zinc foils. These raw materials are converted into layer supports for offset printing plates by suitable operations, such as graining, dull chromium plating, surface oxidation and/or application of an intermediate layer.
Aluminum, which is nowadays perhaps the most frequently used base material for offset printing plates, is surface-roughened according to known methods by dry brushing, wet brushing, sand-blasting or chemical or electrochemical treatment. In order to increase its abrasion resistance, the roughened substrate can additionally be subjected to an anodizing stage in order to build up a thin layer of oxide.
It is customary according to the state of the art to subject anodized support materials of this type to a further stage of treatment in order to improve the adhesion of the light-sensitive layer, to increase their hydrophilic character and/or to facilitate the possibility of developing the light-sensitive copying layers. Methods such as silicatization (German Offenlegungsschrift No. 2,532,769=U.S. Pat. No. 3,902,976) and treatment with complex fluorides (German Auslegeschrift No. 1,300,415=U.S. Pat. No. 3,440,050) or with polyvinylphosphonic acid (German Pat. No. 1,134,093=U.S. Pat. No. 3,276,868; German Pat. No. 1,621,478=U.S. Pat. No. 4,153,461) are described, inter alia, in the patent literature.
However, the methods described above suffer from disadvantages to a greater or lesser extent, so that the support materials thus prepared often no longer meet the present requirements of offset printing. Thus, after treatment with alkali metal silicates, which results in good capacity for development and hydrophilic character, it is necessary to accept a certain impairment of the stability on storage of light-sensitive layers applied thereto.
Although the use of polyvinylphosphonic acid for the after-treatment of supports results in the printing plates having good properties from the point of view of copying and printing technology, the deposition of polyvinylphosphonic acid on the support material can result in difficulties with respect to production technology. These difficulties are caused by Al.sup.3+ cations, which can reach the surface in preceding stages of treatment, principally in the anodizing stage in an electrolyte containing H.sub.2 SO.sub.4 and Al.sub.2 (SO.sub.4).sub.3, and which can only be removed with difficulty and incompletely, even by thorough rinsing. Polyvinylphosphonic acid reacts with these Al.sup.3+ ions to form an extremely sparingly soluble precipitate. This precipitate covers the support in the form of a white dust layer which cannot be removed, even by thorough rinsing. The dust particles can either result in wetting problems at a stage as early as the coating with the light-sensitive layer, or can result later in eruptions of the layer during developing or printing. Problems of this type can only be suppressed by a considerable outlay in terms of production technology.
Support materials which have been treated with polyvinylphosphonic acid tend to be subject to aging phenomena when stored in an uncoated state. These aging phenomena manifest themselves in a decreasing hydrophilic character (a reduction in the ink-repellent action) and in the reduced capacity for development of negative-working, light-sensitive layers which have been applied as a coating only a fairly long time after the preparation of the support.
A further field of use of polyvinylphosphonic acid is as an additive to moistening agents and cleaning agents for the offset printing process, described in German Pat. No. 1,121,632 (=U.S. Pat. No. 3,108,535). Such moistening agents (fountain solutions) must reach the printing plates continuously during the printing process, so that the parts of the plate not to be printed are protected from ink absorption and undesirable water-repellency.